Clamp-equipped load handling devices are well known. Load handling devices may be mounted to load handling vehicles such as power lift trucks for example. Such vehicles include load handling devices having a vertical mast with a clamp movably mounted thereto for upward and downward movement along the length of the mast. These types of clamp-equipped vehicles are often used in the paper and pulp industry. The clamp-equipped vehicle is intended to grip and lift one or more rolls of paper. More particularly, the truck approaches a roll and the clamps are opened so as to engaged the roll therebetween and then closed so as to grip the roll to be moved along the longitudinal length of the mast thereby lifting the paper roll load; the mast can also be simultaneously tilted between left and right directions. The clamping or gripping pressure on paper-rolls, particularly for printing presses, should not be such that the paper-roll will be squeezed to an oval shape during handling of the rolls which includes gripping, lifting and tilting thereof.
Various solutions for avoiding paper-roll ovalization have been proposed. For example, devices for controlling the clamping pressure in response to detected sliding at the gripping surface of the clamps have been provided. A drawback of these devices is that the paper-roll may be damaged during the sliding movement.
Further improvements include electrical load handling devices that monitor the lifting force of the mast via a sensor in order to adapt the clamping force exerted on the roll to the lifting force. The clamping pressure line is provided with a pressure reduction valve that is controlled by a controller (e.g. a computer) linked to the sensor. Thereby, the pressure provided by the clamping pressure line is proportional to the lifting force detected by the sensor. A drawback of such systems is that the lifting pressure may increase as the clamps gripping the paper load move up along the mast, thereby increasing the clamping pressure which may sometimes damage the paper roll. Typically, a regular sized mast can have a first lower section, where the gripping force remains as required and a second upper section, when the lifting force is greater due to the increase in pressure required for lifting a load for an even greater distance which causes the gripping force to be proportionally increased which may damage the roll since the weight of the roll has not changed. Another drawback of such systems is their cost given the fact that a relatively complex and costly array of electrical circuit regulators, sensors and data processors is needed.